Engine system

ABSTRACT

An engine system is provided, including a controller which controls devices of an engine at a given engine speed so that, when a demanded engine load is a first load, a mass ratio (G/F) of intake air inside a cylinder (containing fresh air and burnt gas) to fuel is a first G/F and mixture gas inside the cylinder combusts by flame-propagation, when the demanded load is a second load (&lt;the first load), the G/F is a second G/F (&gt;the first G/F) and an injection center-of-gravity is at a timing such that the entire mixture gas combusts by CI combustion, and when the demanded load is between the first and second loads, the G/F is at a third G/F (between the first and second G/Fs) and the injection center-of-gravity is at a later timing such that at least part of the mixture gas combusts by the CI combustion.

TECHNICAL FIELD

The present disclosure relates to an engine system.

BACKGROUND OF THE DISCLOSURE

Compression ignition combustion (hereinafter, may simply be referred toas “CI combustion”) improves the thermal efficiency of an engine.JP2012-215098A discloses an engine in which a mixture gas is combustedby the CI combustion, more accurately, by HCCI (Homogeneous ChargedCompression Ignition) combustion when an engine load is low, and themixture gas is combusted by SI (Spark Ignition) combustion using a sparkplug when the engine load is high. This engine switches the combustionmode corresponding to the change in the engine load. Note that in the SIcombustion, the mixture gas combusts by flame propagation after theignition, and thus, the SI combustion is synonymous with flamepropagation combustion in the following description.

Meanwhile, the present inventors conducted a diligent study on the HCCIcombustion. As a result, it became apparent that main control factors ofthe HCCI combustion are the temperature of mixture gas inside acylinder, and a mass ratio (G/F) of intake air including burnt gasinside the cylinder to fuel. Moreover, an ignition timing and acombustion period of the HCCI combustion can be controlled by anin-cylinder temperature (T_(IVC)) at a close timing of an intake valveand the G/F being adjusted to be a target T_(IVC) and a target G/F.Furthermore, according to the study, the present inventors found thatthe SI combustion is possible under certain conditions of the T_(IVC)and the G/F, and the HCCI combustion is possible also under certainconditions of the T_(IVC) and the G/F. Moreover, there is a large gapbetween the T_(IVC) and the G/F at which the SI combustion is possible,and the T_(IVC) and the G/F at which the HCCI combustion is possible.

Note that “SI combustion is possible” corresponds to a state where thecombustion stability of the SI combustion meets a standard, and abnormalcombustion can be reduced. For example, when the G/F is too high (i.e.,too lean), the combustion stability of the SI combustion does not meetthe standard. Moreover, when the T_(IVC) is too high, the SI combustionis likely to cause abnormal combustion. Abnormal combustion of the SIcombustion includes preignition and knocking. When the ignition timingis retarded to avoid abnormal combustion, the combustion stability doesnot meet the standard.

“HCCI combustion is possible” corresponds to a state where thecombustion stability of the HCCI combustion meets a standard, andabnormal combustion can be reduced. For example, when the G/F is too low(i.e., too rich), the HCCI combustion is likely to cause abnormalcombustion (e.g., overly rapid combustion). Moreover, also when theT_(IVC) is too high, the HCCI combustion is likely to cause overly rapidcombustion. When the T_(IVC) is too low, the HCCI combustion causesmisfiring and the combustion stability does not meet the standard.

Even if the combustion mode is to be switched between the HCCIcombustion and the SI combustion corresponding to the change in theengine load as disclosed in JP2012-215098A, it is difficult to instantlychange the G/F of the in-cylinder mixture gas to be at the G/Fcorresponding to the switching target combustion mode.

SUMMARY OF THE DISCLOSURE

As a result of further diligent study to solve the above problem, thepresent inventors found a third combustion mode which meets a standardof the combustion stability and is capable of reducing abnormalcombustion at a middle G/F between the G/F at which SI (Spark Ignition)combustion is possible and the G/F at which HCCI (Homogeneous ChargedCompression Ignition) combustion is possible.

When the G/F of a mixture gas is set to the middle G/F at an engine loadbetween a low load for the HCCI combustion and a high load for the SIcombustion, and the third combustion mode is adopted, an amount ofchange in the G/F decreases when the engine load changes between the lowload and the middle load, as well as between the high load and themiddle load. As a result, the engine can seamlessly switch thecombustion mode between the SI combustion, the HCCI combustion, and thethird combustion mode.

According to one aspect of the present disclosure, an engine system isprovided, which includes an engine having a cylinder and a pistonreciprocatably accommodated in the cylinder, an injector attached to theengine and configured to inject fuel into the cylinder, a spark plugattached to the engine and configured to ignite a mixture gas of fueland intake air, the intake air containing fresh air and burnt gas, avariable valve operating device connected to an intake valve and anexhaust valve, and configured to control opening and closing of theintake valve and the exhaust valve to adjust a filling amount of theintake air, and a controller electrically connected to the injector, thespark plug, and the variable valve operating device, and configured tocontrol the injector, the spark plug, and the variable valve operatingdevice according to a demanded load of the engine. When the engineoperates at a given speed and the demanded engine load is a first load,the controller controls the injector and the variable valve operatingdevice to make a mass ratio (G/F) of the intake air inside the cylinderto fuel be at a first G/F, and controls the spark plug so that themixture gas inside the cylinder combusts by flame propagation. When theengine operates at the given speed and the demanded engine load is asecond load lower than the first load, the controller controls theinjector and the variable valve operating device to make the mass ratiobe at a second G/F higher than the first G/F, and controls the injectorto make an injection center of gravity be at a first timing so that theentire mixture gas inside the cylinder combusts by compression ignition,the injection center of gravity being defined based on an injectiontiming and an injection amount of fuel during one cycle. When the engineoperates at the given speed and the demanded engine load is lower thanthe first load and higher than the second load, the controller controlsthe injector and the variable valve operating device to make the massratio be at a third G/F higher than the first G/F and lower than thesecond G/F, and controls the injector to make the injection center ofgravity be at a second timing later than the first timing so that atleast part of the mixture gas inside the cylinder combusts bycompression ignition.

According to this configuration, when the engine operates at the givenspeed and the demanded engine load is the first load, the controllercontrols the injector and the variable valve operating device to makethe mass ratio be at the first G/F, and controls the spark plug. Thespark plug ignites the mixture gas inside the cylinder and this mixturegas combusts by flame propagation (i.e., the SI combustion). Since thefirst G/F is relatively low, the combustion stability of the SIcombustion is enhanced. Moreover, by reducing the burnt gas to make theG/F lower, the in-cylinder temperature decreases and abnormal combustionis reduced.

When the engine operates at the given speed and the demanded engine loadis the second load which is relatively low, the controller controls theinjector and the variable valve operating device to make the mass ratiobe at the second G/F which is relatively high. The injector injects fuelso that the injection center of gravity is at the first timing. Thefirst timing is a relatively early timing. By injecting fuel into thecylinder at the early timing, fuel can be spread using the comparativelystrong intake flow, and thus, a homogeneous or substantially homogeneousmixture gas is formed inside the cylinder. When the demanded engine loadis the second load, the entire mixture gas inside the cylinder combustsby compression ignition (i.e., the HCCI combustion). For example, byincreasing the burnt gas to be introduced into the cylinder to make theG/F higher, the in-cylinder temperature increases, which enhances thecombustion stability of the HCCI combustion. Moreover, the high G/F isadvantageous in improving the fuel efficiency of the engine.

When the engine operates at the given speed and the demanded engine loadis a middle load between the first load and the second load, thecontroller controls the injector and the variable valve operating deviceto make the mass ratio be at the third G/F which is higher than thefirst G/F and lower than the second G/F. The difference between thefirst G/F and the third G/F is small, and the difference between thesecond G/F and the third G/F is also small. By the variable valveoperating device changing the filling amount of the intake aircorresponding to the change in the demanded engine load, the mass ratioof the mixture gas inside the cylinder can be promptly changed betweenthe first G/F and the third G/F or between the second G/F and the thirdG/F.

The injector injects fuel so that the injection center of gravity is atthe second timing which is relatively late. The injector may perform thefuel injection all at once or dividedly. The injection center of gravitymay be defined by the center of mass of fuel injected all at once ordividedly into a plurality of times in one cycle, with respect to acrank angle. When the injection center of gravity is relatively late,the fuel supply into the cylinder is delayed, and thus, a period of timefrom the fuel injection to the ignition of the mixture gas becomesshorter. Unlike the case where the demanded engine load is the secondload as described above, the mixture gas inside the cylinder does notbecome homogeneous. Such an inhomogeneous mixture gas achieves thecombustion which meets the standard of combustion stability whilereducing abnormal combustion at the middle third G/F (in more detail, atleast part of the mixture gas combusts by compression ignition).

Therefore, this engine can seamlessly switch the combustion mode betweenthe SI combustion, the HCCI combustion, and the third combustion mode bypromptly changing the mass ratio of the mixture gas (between the firstG/F, the second G/F, and the third G/F) corresponding to the change inthe engine load. As a result, securing combustion stability and reducingabnormal combustion can be achieved over the entire load range of theengine.

Furthermore, in each of the HCCI combustion and the third combustionmode, at least part of the mixture gas combusts by compression ignition,and also the mass ratio of the mixture gas is comparatively high. Thus,this engine is fuel efficient.

When the engine operates at the given speed and the demanded engine loadis lower than the first load and higher than the second load, thecontroller may inhibit the operation of the spark plug so that theentire mixture gas inside the cylinder combusts by compression ignition.

According to this configuration, the thermal efficiency of the engineimproves.

When the engine operates at the given speed and the demanded engine loadis lower than the first load and higher than the second load, thecontroller may actuate the spark plug so that at least part of themixture gas inside the cylinder combusts by flame propagation, and theremaining mixture gas combusts by compression ignition.

The third combustion mode may be SPCCI (SPark Controlled CompressionIgnition) combustion, as one example. Since in the SPCCI combustion, thetiming of the compression ignition can be controlled by adjusting thespark ignition timing. Therefore, the SPCCI combustion is advantageousin both securing combustion stability and reducing abnormal combustion.Moreover, the SPCCI combustion improves the thermal efficiency of theengine compared to the SI combustion.

When the engine operates at the given speed and the demanded engine loadis a third load lower than the first load and higher than the secondload, the controller may inhibit the operation of the spark plug so thatthe entire mixture gas inside the cylinder combusts by compressionignition. When the engine operates at the given speed and the demandedengine load is a fourth load lower than the first load and higher thanthe third load, the controller may actuate the spark plug so that atleast part of the mixture gas inside the cylinder combusts by flamepropagation, and the remaining mixture gas combusts by compressionignition.

According to this configuration, when the demanded engine load isrelatively low, the controller controls the spark plug to not actuate sothat the entire mixture gas combusts by compression ignition. Bothsecuring combustion stability and reducing abnormal combustion can beachieved when the mixture gas is at the middle G/F. When the demandedengine load is relatively high, the fuel amount increases. Thecontroller actuates the spark plug so that at least part of the mixturegas inside the cylinder combusts by flame propagation, and the remainingmixture gas combusts by compression ignition. The SPCCI combustionenables both securing combustion stability and reducing abnormalcombustion when the demanded engine load is relatively high.

When the engine operates at the given speed and the demanded engine loadis the fourth load, the controller may retard the injection center ofgravity compared to the injection center of gravity when the demandedengine load is the third load.

According to this, when the demanded engine load is high, abnormalcombustion is reduced by retarding the injection timing of fuel.Moreover, the spark plug ignites the mixture gas to accelerate thecombustion of the mixture gas, which secures combustion stability.

When the engine operates at the given speed and the demanded engine loadis the second load, the controller may control the injector to injectfuel during an intake stroke. When the engine operates at the givenspeed and the demanded engine load is lower than the first load andhigher than the second load, the controller may control the injector toinject fuel during each of the intake stroke and a compression stroke.

According to this configuration, when the engine operates at the givenspeed and the demanded engine load is the second load, the injectorinjects fuel into the cylinder during the intake stroke. The fuel can bespread using the intake flow, and thus, the mixture gas formed insidethe cylinder becomes homogeneous or substantially homogeneous. Thehomogeneous mixture gas inside the cylinder combusts by compressionignition (i.e., the HCCI combustion).

When the engine operates at the given speed and the demanded engine loadis the middle load lower than the first load and higher than the secondload, the injector injects fuel into the cylinder during each of theintake stroke and the compression stroke. The fuel injected during theintake stroke spreads using the intake flow, and forms the homogeneousmixture gas. The fuel then injected during the compression stroke makesthe mixture gas inhomogeneous. The inhomogeneous mixture gas achievesthe combustion which meets the standard of combustion stability whilereducing abnormal combustion.

When the engine operates at the given speed and the demanded engine loadis lower than the first load and higher than the second load, thecontroller may switch between a first injection mode and a secondinjection mode based on an estimated G/F, and an estimated temperatureinside the cylinder at a close timing of the intake valve, the firstinjection mode being a mode in which the controller controls theinjector to inject fuel during each of the intake stroke and a middleperiod of the compression stroke, and the second injection mode being amode in which the controller controls the injector to inject fuel duringeach of the intake stroke and an end period of the compression stroke.

In the first injection mode, the in-cylinder temperature decreases dueto latent heat during vaporization of fuel injected into the cylinder inthe middle period of the compression stroke. By the in-cylindertemperature locally decreasing, the ignition timing of the mixture gasinside the cylinder is retarded and the mixture gas is compressed to beignited near the top dead center of the compression stroke. Then, themixture gas combusts comparatively slowly. The first injection mode isadvantageous in reducing abnormal combustion.

In the second injection mode, the injector injects fuel into thecylinder in the end period of the compression stroke. The injected fuelis difficult to spread due to the high in-cylinder pressure, and a lumpof mixture gas at a high fuel concentration is formed. This lump ofmixture gas accelerates the compression ignition, and the mixture gasinside the cylinder is promptly combusted. The second injection mode isadvantageous in improving combustion stability.

A cavity may be formed in a top surface of the piston. In the firstinjection mode, the controller may control the injector to inject fuelto outside of the cavity in the middle period of the compression strokesuch that an amount of injection during the compression stroke is largerthan an amount of injection during the intake stroke. In the secondinjection mode, the controller may control the injector to inject fuelto inside of the cavity in the end period of the compression stroke suchthat an amount of injection during the compression stroke is smallerthan an amount of injection during the intake stroke.

In the first injection mode, the fuel injected into the cylinder fromthe injector reaches an area outside of the cavity. The area outside thecavity is originally low in the temperature since it is located near acylinder liner, and as described above, the temperature furtherdecreases due to latent heat during vaporization of fuel.

The injection amount of fuel during the compression stroke is largerthan the injection amount during the intake stroke. Since the areaoutside the cavity is large, generation of smoke can be reduced evenwhen the amount of fuel reached inside the area is large. Thein-cylinder temperature decreases as the amount of fuel increases. Theinjection amount of fuel during the compression stroke may be set to anamount capable of achieving a demanded decrease in the temperature.

Thus, in the first injection mode, a comparatively slow combustion isachieved and abnormal combustion is reduced.

In the second injection mode, the fuel injected into the cylinder fromthe injector reaches an area inside the cavity. The area inside thecavity is comparatively high in the temperature since it is far from thecylinder liner. The lump of mixture gas at a high fuel concentration isformed inside the cavity at the high temperature, the compressionignition is accelerated.

The injection amount of fuel during the compression stroke is smallerthan the injection amount of fuel during the intake stroke. Since thefuel injection during the compression stroke is performed in the endperiod of the compression stroke as described above, the injected fuelstays inside the cavity and is difficult to spread. Since the fuelamount is small, the generation of smoke can be suppressed. Theinjection amount of fuel during the compression stroke can be set to anamount capable of achieving both of the demanded acceleration of thecompression ignition and the reduction in the generation of smoke.

When the engine operates at the given speed and the demanded engine loadis the first load, the controller may control the injector to injectfuel during the intake stroke.

When the injector injects fuel into the cylinder during the intakestroke, the fuel can be spread using the intake flow, and the mixturegas inside the cylinder becomes homogeneous or substantiallyhomogeneous. The spark plug ignites the homogeneous mixture gas and thismixture gas combusts by flame propagation. Both securing combustionstability and reducing abnormal combustion can be achieved.

When the engine operates at the given speed and the demanded engine loadis the first load, the controller may control the injector to injectfuel in a latter half of the compression stroke.

Retarding the injection timing of fuel by the injector can reduceabnormal combustion. When injecting fuel in the latter half of thecompression stroke, this injection generates a flow inside the cylinder,and then when the spark plug ignites the mixture gas, the flow generatedby the injection accelerates the flame propagation. The combustionstability of the SI combustion also improves.

The variable valve operating device may control the opening and closingof the intake valve and the exhaust valve so that the burnt gas remainsinside the cylinder, or the burnt gas is introduced into the cylinderthrough the intake valve or the exhaust valve.

By making so-called internal exhaust gas recirculation (EGR) gas remaininside or be introduced into the cylinder, the in-cylinder temperaturecan be increased, which is advantageous in improving the combustionstability of the compression ignition combustion.

A geometric compression ratio of the engine may be 15:1 or above.

High geometric compression ratio is advantageous in improving thecombustion stability of the compression ignition combustion. Moreover,the high geometric compression ratio improves the thermal efficiency ofthe engine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating an engine system.

FIG. 2 is a view illustrating a structure of a combustion chamber of anengine, where an upper part of this figure is a plan view of thecombustion chamber, and a lower part of this figure is a cross-sectionalview taken along a line II-II in the upper part when fuel is injectedinto a cylinder in a middle period of a compression stroke.

FIG. 3 is a block diagram of the engine system.

FIG. 4 is a view illustrating a base map related to operation of theengine.

FIG. 5 is a view illustrating open and close operations of an intakevalve and an exhaust valve, an injection timing of fuel, and an ignitiontiming in each combustion mode.

FIG. 6 is a view illustrating a state where fuel is injected into thecylinder in an end period of the compression stroke.

FIG. 7 is a view illustrating a definition of an injection center ofgravity.

FIG. 8 illustrates a modification of the open and close operations ofthe intake valve and the exhaust valve in each combustion mode.

FIG. 9 is a view illustrating ranges defined based on a G/F and aT_(IVC), within which each combustion mode is achieved.

FIG. 10 is a view illustrating a selection map of the combustion mode ina low-load range where HCCI combustion is performed.

FIG. 11 is a flowchart illustrating control process related to theoperation of the engine, executed by an engine control unit (ECU).

DETAILED DESCRIPTION OF THE DISCLOSURE

Hereinafter, one embodiment of a method of controlling an engine and anengine system is described with reference to the accompanying drawings.The engine, the engine system, and the control method thereof are merelyillustration.

FIG. 1 is a view illustrating the engine system. FIG. 2 is a viewillustrating a structure of a combustion chamber of the engine. Theintake side and the exhaust side illustrated in FIG. 1 are opposite fromthe intake side and the exhaust side illustrated in FIG. 2 . FIG. 3 is ablock diagram illustrating a control device for the engine.

The engine system includes an engine 1. The engine 1 includes cylinders11, and is a four-stroke engine in which an intake stroke, a compressionstroke, an expansion stroke, and an exhaust stroke are repeated in eachcylinder 11. The engine 1 is mounted on a four-wheeled automobile, andthe automobile travels according to the operation of the engine 1. Fuelof the engine 1 is gasoline in this example.

(Configuration of Engine)

The engine 1 is provided with a cylinder block 12 and a cylinder head13. The cylinder head 13 is placed on the cylinder block 12. A pluralityof cylinders 11 are formed inside the cylinder block 12. The engine 1 isa multi-cylinder engine. In FIG. 1 , only one cylinder 11 isillustrated.

A piston 3 is inserted into each cylinder 11. The piston 3 is coupled toa crankshaft 15 through a connecting rod 14. The piston 3 reciprocatesinside the cylinder 11. The piston 3, the cylinder 11, and the cylinderhead 13 define a combustion chamber 17.

As illustrated in the lower part of FIG. 2 , a lower surface of thecylinder head 13 (i.e., a ceiling of the cylinder 11) is constituted bya sloped surface 1311 and a sloped surface 1312. The sloped surface 1311is a slope on a side of an intake valve 21 (described later), andinclines upwardly toward the central part of the ceiling of the cylinder11. The sloped surface 1312 is a slope on a side of an exhaust valve 22(described later), and inclines upwardly toward the central part of theceiling of the cylinder 11. The ceiling of the cylinder 11 is aso-called pentroof-type.

A cavity 31 is formed in a top surface of the piston 3. The cavity 31 isdented from the top surface of the piston 3. The cavity 31 has a shallowdish shape in this example. The central part of the cavity 31 protrudesupwardly, and the protruded part has a substantially conical shape.

A geometric compression ratio of the engine 1 is set to 15:1 or higher,and set to, for example, 30:1 or lower. As will be described later, theengine 1 performs compression ignition (CI) combustion of a mixture gasin part of an operation range of the engine. The CI combustion can bestabilized by a comparatively high geometric compression ratio.

The cylinder head 13 is formed with intake ports 18 for the respectivecylinders 11 such that each intake port 18 communicates with the insideof the cylinder 11. Although detailed illustration is omitted, theintake port 18 is a so-called tumble port. That is, the intake port 18has a shape which generates a tumble flow inside the cylinder 11. Thepentroof-type ceiling of the cylinder 11 and the tumble port generatethe tumble flow inside the cylinder 11. Note that the intake port 18includes two intake ports in this example.

Each intake port 18 is provided with the intake valve 21 which opens andcloses the intake port 18. A valve operating device is connected to theintake valve 21, and opens and closes the intake valve 21 at a giventiming. The valve operating device may be a variable valve operatingdevice which varies a valve timing and/or a valve lift. As illustratedin FIG. 3 , the valve operating device includes an intake S-VT(Sequential-Valve Timing) 231 of a hydraulic type or an electric type.The intake S-VT 231 continuously changes a rotational phase of an intakecamshaft within a given angle range.

The valve operating device also includes an intake CVVL (ContinuouslyVariable Valve Lift) 232. As illustrated in FIG. 5 , the intake CVVL 232can continuously change the lift amount of the intake valve 21 within agiven range. Various known configurations can be adopted for the intakeCVVL 232. For example, as disclosed in JP2007-085241A, the intake CVVL232 may be comprised of a linkage mechanism, a control arm, and astepping motor. The linkage mechanism reciprocatably pivots a cam whichoperates the intake valve 21, in an interlocking manner with a rotationof a camshaft. The control arm variably sets a lever ratio of thelinkage mechanism. As the lever ratio of the linkage mechanism changes,a pivoting amount of the cam which pushes down the intake valve 21changes. The stepping motor electrically drives the control arm tochange the pivoting amount of the cam, thus changing the lift amount ofthe intake valve 21.

The cylinder head 13 is formed with exhaust ports 19 for the respectivecylinders 11 such that each exhaust port 19 communicates with the insideof the cylinder 11. Note that the exhaust port 19 includes two exhaustports in this example.

Each exhaust port 19 is provided with the exhaust valve 22 which opensand closes the exhaust port 19. A valve operating device is connected tothe exhaust valve 22, and opens and closes the exhaust valve 22 at agiven timing. The valve operating device may be a variable valveoperating device which varies a valve timing and/or a valve lift. Asillustrated in FIG. 3 , the valve operating device includes an exhaustS-VT (Sequential-Valve Timing) 241 of a hydraulic type or an electrictype. The exhaust S-VT 241 continuously changes a rotational phase of anexhaust camshaft within a given angle range.

The valve operating device also includes an exhaust VVL (Variable ValveLift) 242. Although illustration is omitted, the exhaust VVL 242 canswitch a cam which opens and closes the exhaust valve 22. Various knownconfigurations can be adopted for the exhaust VVL 242. For example, asdisclosed in JP2018-168796A, the exhaust VVL 242 may be comprised of afirst cam, a second cam, and a switching mechanism which switchesbetween the first cam and the second cam. The first cam opens and closesthe exhaust valve 22 during an exhaust stroke. The second cam opens andcloses the exhaust valve 22 during the exhaust stroke, and also opensand closes the exhaust valve 22 again during an intake stroke, asillustrated in FIG. 5 . The exhaust VVL 242 can change the lift of theexhaust valve 22 by changing the cam to open and close the exhaust valve22 between the first cam and the second cam.

The intake S-VT 231, the intake CVVL 232, the exhaust S-VT 241, and theexhaust VVL 242 control the opening and closing of the intake valve 21and the exhaust valve 22 to adjust an amount of fresh air and an amountof burnt gas to be introduced into the cylinder 11. The intake S-VT 231,the intake CVVL 232, the exhaust S-VT 241, and the exhaust VVL 242adjust a filling amount of intake air.

Injectors 6 are attached to the cylinder head 13 for the respectivecylinders 11. As illustrated in FIG. 2 , each injector 6 is provided tothe central part of the cylinder 11 in the plan view. In detail, theinjector 6 is disposed in a valley part of the pentroof where the slopedsurface 1311 and the sloped surface 1312 intersect with each other.

The injector 6 directly injects fuel into the cylinder 11. The injector6 is a multiple nozzle hole type having a plurality of nozzle holes (notillustrated in detail). As illustrated by two-dot chain lines in FIG. 2, the injector 6 injects fuel radially outwardly from the central partto a peripheral part of the cylinder 11. As illustrated in the lowerpart of FIG. 2 , an axis of the nozzle hole of the injector 6 has agiven angle θ with respect to the center axis X of the cylinder 11.Although in this example the injector 6 has ten nozzle holes which aredisposed at an equal angle in a circumferential direction, the number ofnozzle holes and the positions thereof are not particularly limited tothis structure.

The injector 6 is connected to a fuel supply system 61. The fuel supplysystem 61 is comprised of a fuel tank 63 which stores fuel, and a fuelsupply passage 62 which couples the fuel tank 63 to the injector 6. Afuel pump 65 and a common rail 64 are interposed in the fuel supplypassage 62. The fuel pump 65 pumps fuel to the common rail 64. The fuelpump 65 is a plunger-type pump driven by the crankshaft 15 in thisexample. The common rail 64 stores at a high fuel pressure the fuelpumped from the fuel pump 65. When the injector 6 is valve-opened, thefuel stored in the common rail 64 is injected into the cylinder 11 fromthe nozzle holes of the injector 6. The pressure of the fuel supplied tothe injector 6 may be changed according to the operating state of theengine 1. Note that the configuration of the fuel supply system 61 isnot limited to the configuration described above.

A first spark plug 251 and a second spark plug 252 are attached to thecylinder head 13 for each cylinder 11. Each of the first spark plug 251and the second spark plug 252 forcibly ignites the mixture gas insidethe cylinder 11. As illustrated in FIG. 2 , the first spark plug 251 isdisposed between the two intake valves 21, and the second spark plug 252is disposed between the two exhaust valves 22. A tip end of the firstspark plug 251 and a tip end of the second spark plug 252 are locatednear the ceiling of the cylinder 11 on the intake side and the exhaustside of the injector 6, respectively. Note that only one spark plug maybe provided.

The engine 1 is connected at one side surface to an intake passage 40.The intake passage 40 communicates with the intake ports 18 of thecylinders 11. Air to be introduced into the cylinders 11 flows throughthe intake passage 40. The intake passage 40 is provided at itsupstream-end part with an air cleaner 41. The air cleaner 41 filters theair. The intake passage 40 is provided, near its downstream end, with asurge tank 42. Part of the intake passage 40 downstream of the surgetank 42 constitutes independent passages branching for the respectivecylinders 11. Downstream ends of the independent passages are connectedto the intake ports 18 of the cylinders 11, respectively.

The intake passage 40 is provided, between the air cleaner 41 and thesurge tank 42, with a throttle valve 43. The throttle valve 43 adjustsits opening to control an amount of air to be introduced into thecylinder 11. Basically, the throttle valve 43 is fully opened during theoperation of the engine 1. The introducing amount of air is controlledby the variable valve operating device described above.

The engine 1 is provided with a swirl generator which generates a swirlflow inside the cylinders 11. The swirl generator has a swirl controlvalve 56 attached to the intake passage 40. Although not illustrated indetail, the intake passage 40 includes a primary passage and a secondarypassage, which are connected to each cylinder 11 downstream of the surgetank 42 corresponding to the two intake ports 18, and the swirl controlvalve 56 is provided to the secondary passage. The swirl control valve56 is an opening control valve which is capable of choking across-section of the secondary passage. When the opening of the swirlcontrol valve 56 is small, a flow rate of intake air flowing into thecylinder 11 from the primary passage is relatively large while a flowrate of intake air flowing into the cylinder 11 from the secondarypassage is relatively small, which increases the swirl flow inside thecylinder 11. On the other hand, when the opening of the swirl controlvalve 56 is large, the flow rate of intake air flowing into the cylinder11 from the primary passage and the flow rate of intake air flowing fromthe secondary passage are substantially equal, which reduces the swirlflow inside the cylinder 11. When the swirl control valve 56 is fullyopened, the swirl flow is not generated.

The engine 1 is connected at the other side surface to an exhaustpassage 50. The exhaust passage 50 communicates with the exhaust ports19 of the cylinders 11. The exhaust passage 50 is a passage throughwhich exhaust gas discharged from the cylinders 11 flows. Althoughdetailed illustration is omitted, an upstream part of the exhaustpassage 50 constitutes independent passages branching for the respectivecylinders 11. Upstream ends of the independent passages are connected tothe exhaust ports 19 of the cylinders 11, respectively.

The exhaust passage 50 is provided with an exhaust gas purificationsystem having a plurality of catalytic converters. An upstream catalyticconverter includes, for example, a three-way catalyst 511 and a GPF(Gasoline Particulate Filter) 512. A downstream catalytic converterincludes a three-way catalyst 513. Note that the exhaust gaspurification system is not limited to the illustrated configuration. Forexample, the GPF may be omitted. Moreover, the catalytic converter isnot limited to the one including the three-way catalyst. Further, thedisposed order of the three-way catalyst and the GPF may be changedsuitably.

An exhaust gas recirculation (EGR) passage 52 is connected between theintake passage 40 and the exhaust passage 50. The EGR passage 52 is apassage through which part of exhaust gas recirculates to the intakepassage 40. An upstream end of the EGR passage 52 is connected to partof the exhaust passage 50 between the upstream and downstream catalyticconverters. A downstream end of the EGR passage 52 is connected to partof the intake passage 40 between the throttle valve 43 and the surgetank 42.

The EGR passage 52 is provided with an EGR cooler 53 of a water-cooledtype. The EGR cooler 53 cools exhaust gas. The EGR passage 52 is alsoprovided with an EGR valve 54. The EGR valve 54 adjusts a flow rate ofexhaust gas flowing through the EGR passage 52. The EGR valve 54 changesits opening to adjust a recirculating amount of the cooled exhaust gas.

As illustrated in FIG. 3 , the control device for the engine 1 isprovided with an ECU (engine control unit) 10 to operate the engine 1.The ECU 10 is a controller based on a well-known microcomputer, andincludes a processor (e.g., a central processing unit (CPU)) 101, memory102, and an interface (I/F) circuit 103. The processor 101 executes aprogram. The memory 102 is comprised of, for example, RAM (Random AccessMemory) and/or ROM (Read Only Memory), and stores the program and data.The I/F circuit 103 outputs and inputs an electric signal. The ECU 10 isone example of a “controller.”

As illustrated in FIGS. 1 and 3 , various kinds of sensors SW1-SW10 areconnected to the ECU 10. The sensors SW1-SW10 output signals to the ECU10. The sensors include the following sensors. An airflow sensor SW1 isprovided to the intake passage 40 downstream of the air cleaner 41, andmeasures the flow rate of air flowing through the intake passage 40. Anintake temperature sensor SW2 is provided to the intake passage 40downstream of the air cleaner 41, and measures the temperature of theair flowing through the intake passage 40. An intake pressure sensor SW3is attached to the surge tank 42, and measures the pressure of the airto be introduced into the cylinder 11. An in-cylinder pressure sensorSW4 is attached to the cylinder head 13 for each cylinder 11, andmeasures the pressure inside the cylinder 11. A water temperature sensorSW5 is attached to the engine 1, and measures the temperature ofcoolant. A crank angle sensor SW6 is attached to the engine 1, andmeasures a rotational angle of the crankshaft 15. An accelerator openingsensor SW7 is attached to an accelerator pedal mechanism, and measuresan accelerator opening corresponding to an operation amount of anaccelerator pedal. An intake cam-angle sensor SW8 is attached to theengine 1, and measures a rotational angle of the intake camshaft. Anexhaust cam-angle sensor SW9 is attached to the engine 1, and measures arotational angle of the exhaust camshaft. An intake cam-lift sensor SW10is attached to the engine 1, and measures the lift amount of the intakevalves 21.

The ECU 10 determines the operating state of the engine 1 based on thesignals of the sensors SW1-SW10, and also calculates a control amount ofeach device based on a control logic set in advance. The control logicis stored in the memory 102. The control logic includes calculating atarget amount and/or the control amount by using a map stored in thememory 102.

The ECU 10 outputs electric signals related to the calculated controlamounts to the injector 6, the first spark plug 251, the second sparkplug 252, the intake S-VT 231, the intake CVVL 232, the exhaust S-VT241, the exhaust VVL 242, the fuel supply system 61, the throttle valve43, the EGR valve 54, and the swirl control valve 56.

(Operation Control Map of Engine)

FIG. 4 illustrates a base map related to the control of the engine 1.The base map is stored in the memory 102 of the ECU 10. The base mapincludes a first base map 401 and a second base map 402. The ECU 10uses, for controlling the engine 1, the base map which is selected fromthe two base maps based on the temperature (high or low) of the coolantof the engine 1. The first base map 401 is a base map when the engine 1is warm (warm state), and the second base map 402 is a base map when theengine 1 is cold (cold state).

The first base map 401 and the second base map 402 are defined based ona load and a speed of the engine 1. The first base map 401 is roughlydivided into four ranges, a first range, a second range, a third range,and a fourth range, according to the load and speed. In more detail, thefirst range includes a high-speed range 411, and ahigh-load/middle-speed range 412. The high-speed range 411 covers from alow-load range to a high-load range. The second range corresponds tohigh-load/low-speed ranges 413 and 414. The third range corresponds to alow-load range 415 including idling operation, and covers from alow-speed range to a middle-speed range. The fourth range corresponds tomiddle-load ranges 416 and 417 where the load is higher than thelow-load range 415, and lower than the high-load/middle-speed range 412and the high-load/low-speed ranges 413 and 414.

The high-load/low-speed ranges 413 and 414 are comprised of a firsthigh-load/low-speed range 413 at a relatively low load, and a secondhigh-load/low-speed range 414 at a load higher than the firsthigh-load/low-speed range 413 and including the maximum load. Themiddle-load ranges 416 and 417 are comprised of a first middle-loadrange 416, and a second middle-load range 417 at a load lower than thefirst middle-load range 416.

The second base map 402 is divided into three ranges, a first range, asecond range, and a third range. In more detail, the first rangeincludes a high-speed range 421 and a high-load/middle-speed range 422.The second range corresponds to high-load/low-speed ranges 423 and 424.The third range corresponds to a low-and-middle load range 425 covering,in the load direction, from a low-load range including the idlingoperation to a middle-load range, and in the speed direction, from alow-speed range to a middle-speed range.

The high-load/low-speed ranges 423 and 424 are comprised of a firsthigh-load/low-speed range 423 at a relatively low load, and a secondhigh-load/low-speed range 424 at a load higher than the firsthigh-load/low-speed range 423 and including the maximum load.

The first range of the second base map 402 corresponds to the firstrange of the first base map 401, the second range of the second base map402 corresponds to the second range of the first base map 401, and thethird range of the second base map 402 corresponds to the third rangeand the fourth range of the first base map 401.

Here, the low-speed range, the middle-speed range, and the high-speedrange may correspond to a low-speed range, a middle-speed range, and ahigh-speed range when the entire operation range of the engine 1 issubstantially equally divided in the speed direction into three,respectively.

Moreover, the low-load range, the middle-load range, and the high-loadrange may correspond to a low-load range, a middle-load range, and ahigh-load range when the entire operation range of the engine 1 issubstantially equally divided in the load direction into three,respectively.

(Combustion Mode of Engine)

Next, the operation of the engine 1 in each range is described indetail. The ECU 10 changes the open and close operations of the intakevalve 21 and the exhaust valve 22, the injection timing of fuel, andwhether or not to perform the ignition, according to a demanded load andthe speed of the engine 1. A combustion mode of the mixture gas insidethe cylinder 11 is changed by the filling amount of intake air, theinjection timing of fuel, and whether or not to perform the ignitionbeing changed. The combustion mode of the engine 1 changes betweenhomogeneous SI combustion, retarded SI combustion, HCCI (HomogeneousCharged Compression Ignition) combustion, SPCCI (SPark ControlledCompression Ignition) combustion, and MPCI (Multiple Premixed fuelinjection Compression Ignition) combustion. FIG. 5 illustrates the openand close operations of the intake valve 21 and the exhaust valve 22,the injection timing of fuel, the ignition timing, and a waveform of aheat release rate which is generated inside the cylinder 11 by thecombustion of the mixture gas in each combustion mode. In FIG. 5 , acrank angle progresses from left to right. Below, each combustion modein the warm state of the engine 1 is described as one example.

1. Homogeneous SI Combustion

When the engine 1 operates in the first range (i.e., in the high-speedrange 411 or the high-load/middle-speed range 412), the ECU 10 combuststhe mixture gas inside the cylinder 11 by flame propagation. In moredetail, the intake S-VT 231 sets the open and close timings of theintake valve 21 to given timings. The intake CVVL 232 sets the liftamount of the intake valve 21 to a given lift amount. The lift amount ofthe intake valve 21 is substantially the same as the lift amount of theexhaust valve 22 (described later). The exhaust S-VT 241 sets the openand close timings of the exhaust valve 22 to given timings. The intakevalve 21 and the exhaust valve 22 both open near an intake top deadcenter (TDC) (see 701). The exhaust VVL 242 opens and closes the exhaustvalve 22 only once. According to this open-and-close mode of the intakevalve 21 and the exhaust valve 22, a comparatively large amount of freshair, and a comparatively small amount of burnt gas are introduced intothe cylinder 11. Basically, the burnt gas is internal EGR gas whichremains inside the cylinder 11.

The injector 6 injects fuel into the cylinder 11 during an intake stroke(see 702). The injector 6 may inject fuel all at once as illustrated inFIG. 5 . The fuel injected into the cylinder 11 is spread by a strongintake flow, and the mixture gas at a homogeneous fuel concentration isformed inside the cylinder 11. A mass ratio of the mixture gas (i.e., amass ratio G/F of intake air inside the cylinder 11 containing burnt gasto fuel) is about 20:1. Note that a mass ratio A/F of fresh air insidethe cylinder 11 to fuel is a stoichiometric air fuel ratio.

The first spark plug 251 and the second spark plug 252 are both actuatedto ignite the mixture gas near a compression TDC (see 703). The firstspark plug 251 and the second spark plug 252 may ignite the mixture gassimultaneously or at different timings.

After the ignition by the first spark plug 251 and the second spark plug252, the mixture gas combusts by flame propagation (see 704). In thehigh-speed range 411 where the speed is too high for the CI combustion,and in the high-load/middle-speed range 412 where the load is too highfor the CI combustion, the engine 1 can operate while securingcombustion stability and reducing abnormal combustion.

Since the homogeneous mixture gas is combusted by jump spark ignition inthis combustion mode, this mode may be referred to as the “homogeneousSI combustion.”

2. Retarded SI Combustion

When the engine 1 operates in the second range (i.e., in the firsthigh-load/low-speed range 413 or the second high-load/low-speed range414), the ECU 10 combusts the mixture gas inside the cylinder 11 byflame propagation. In more detail, when the engine 1 operates in thesecond high-load/low-speed range 414, the intake S-VT 231 sets the openand close timings of the intake valve 21 to given timings. The intakeCVVL 232 sets the lift amount of the intake valve 21 to a given liftamount. The lift amount of the intake valve 21 is substantially the sameas the lift amount of the exhaust valve 22 (described later). Theexhaust S-VT 241 sets the open and close timings of the exhaust valve 22to given timings. The intake valve 21 and the exhaust valve 22 both opennear the intake TDC (see 705). The exhaust VVL 242 opens and closes theexhaust valve 22 only once. According to this open-and-close mode of theintake valve 21 and the exhaust valve 22, a comparatively large amountof fresh air, and a comparatively small amount of burnt gas areintroduced into the cylinder 11. Basically, the burnt gas is theinternal EGR gas which remains inside the cylinder 11. The G/F is about20:1.

When the engine 1 operates in the first high-load/low-speed range 413,the intake S-VT 231 sets the open and close timings of the intake valve21 to given timings. The intake CVVL 232 sets the lift amount of theintake valve 21 to be smaller than the lift amount in the secondhigh-load/low-speed range 414. The close timing of the intake valve 21in the first high-load/low-speed range 413 is advanced from the closetiming in the second high-load/low-speed range 414 (see 709). Theexhaust S-VT 241 sets the open and close timings of the exhaust valve 22to given timings. The intake valve 21 and the exhaust valve 22 both opennear the intake TDC. The exhaust VVL 242 opens and closes the exhaustvalve 22 only once. According to this open-and-close mode of the intakevalve 21 and the exhaust valve 22, the amount of fresh air introducedinto the cylinder 11 decreases and the amount of burnt gas increases,compared to the mode in the second high-load/low-speed range 414. TheG/F in the first high-load/low-speed range 413 is about 25:1, which isleaner than the G/F in the second high-load/low-speed range 414.

Since the load is high and the speed is low in the firsthigh-load/low-speed range 413 and the second high-load/low-speed range414, abnormal combustion (e.g., preignition and knocking) easily occurs.The injector 6 injects fuel into the cylinder 11 during the compressionstroke (see 706 and 710). By retarding the timing of injecting fuel intothe cylinder 11, abnormal combustion can be reduced. The injector 6 mayinject fuel all at once as illustrated in FIG. 5 .

In the second high-load/low-speed range 414 where the load is relativelyhigh, the injector 6 injects fuel into the cylinder 11 at a relativelylate timing (see 706). The injector 6 may inject fuel, for example, in alatter half of the combustion stroke or an end period of the compressionstroke. Note that the latter half of the compression stroke correspondsto a latter half when the compression stroke is equally divided intotwo, an early half and a latter half. The end period of the compressionstroke corresponds to an end period when the compression stroke isequally divided into three, an early period, a middle period, and an endperiod. In the second high-load/low-speed range 414 where the load isrelatively high, retarding the injection timing of fuel is advantageousto reduce abnormal combustion.

In the first high-load/low-speed range 413 where the load is relativelylow, the injector 6 injects fuel into the cylinder 11 at a relativelyearly timing (see 710). The injector 6 may inject fuel, for example, inthe middle period of the compression stroke. The middle period of thecompression stroke corresponds to the middle period when the compressionstroke is equally divided into three, the early period, the middleperiod, and the end period.

The fuel injected into the cylinder 11 during the compression stroke isspread by the injection flow. An injection pressure of fuel is preferredto be higher in order to rapidly combust the mixture gas so thatabnormal combustion is reduced and combustion stability is improved. Thehigh injection pressure generates a strong flow inside the cylinder 11at a high pressure near the compression TDC. The strong flow acceleratesthe flame propagation.

The first spark plug 251 and the second spark plug 252 both ignite themixture gas near the compression TDC (see 707 and 711). The first sparkplug 251 and the second spark plug 252 may ignite the mixture gassimultaneously or at different timings. In the secondhigh-load/low-speed range 414 where the load is relatively high, thefirst spark plug 251 and the second spark plug 252 perform the ignitionat a timing later than the compression TDC in accordance with theretarded injection timing of fuel. After the ignition by the first sparkplug 251 and the second spark plug 252, the mixture gas combusts byflame propagation (see 708 and 712).

When the engine 1 is in the operating state where the speed is low andabnormal combustion easily occurs, the engine 1 can operate whilesecuring combustion stability and reducing abnormal combustion. Sincethe injection timing is retarded in this combustion mode, thiscombustion mode may be referred to as the “retarded SI combustion.”Specifically, the combustion mode in the first high-load/low-speed range413 may be referred to as a “first retarded SI combustion,” and thecombustion mode in the second high-load/low-speed range 414 may bereferred to as a “second retarded SI combustion.”

3. HCCI Combustion

When the engine 1 operates in the third range (i.e., in the low-loadrange 415), the ECU 10 combusts the mixture gas inside the cylinder 11by compression ignition. In more detail, when the engine 1 operates inthe low-load range 415, the exhaust VVL 242 opens and closes the exhaustvalve 22 twice. That is, the exhaust VVL 242 switches the first camto/from the second cam according to the change in the operation rangebetween the first range and the second range, and the third range. Theexhaust valve 22 is opened and closed during the exhaust stroke, andopened and closed also during the intake stroke. The exhaust S-VT 241sets the open and close timings of the exhaust valve 22 to giventimings. The intake S-VT 231 retards the open and close timings of theintake valve 21. The intake CVVL 232 sets the lift amount of the intakevalve 21 to be small. The intake valve 21 is closed at the most retardedtiming (see 713).

According to this open-and-close mode of the intake valve 21 and theexhaust valve 22, a comparatively small amount of fresh air and a largeamount of burnt gas are introduced into the cylinder 11. Basically, theburnt gas is the internal EGR gas which remains inside the cylinder 11.The G/F of the mixture gas is about 40:1. The large amount of internalEGR gas introduced into the cylinder 11 increases the in-cylindertemperature.

The injector 6 injects fuel into the cylinder 11 during the intakestroke (see 714). As described above, the fuel is spread by the strongintake flow, and the homogeneous mixture gas is formed inside thecylinder 11. The injector 6 may inject fuel all at once as illustratedin FIG. 5 . Alternatively, the injector 6 may inject fuel dividedly(split injection).

When the engine 1 operates in the low-load range 415, the first sparkplug 251 and the second spark plug 252 do not perform the ignition. Themixture gas inside the cylinder 11 is compressed and ignited near thecompression TDC (see 715). Since the load of the engine 1 is low and thefuel amount is small, by making the G/F lean, the CI combustion (moreaccurately, the HCCI combustion) can be achieved while abnormalcombustion is reduced. Moreover, by introducing a large amount ofinternal EGR gas and increasing the in-cylinder temperature, thestability of the HCCI combustion and thermal efficiency of the engine 1improve.

4. SPCCI Combustion

When the engine 1 operates in the fourth range (in detail, in the firstmiddle-load range 416), the ECU 10 combusts part of mixture gas insidethe cylinder 11 by flame propagation, and combusts the remaining mixturegas by compression ignition. In more detail, the exhaust S-VT 241 setsthe open and close timings of the exhaust valve 22 to given timings. Theexhaust VVL 242 opens and closes the exhaust valve 22 twice (see 716).Internal EGR gas is introduced into the cylinder 11. The intake CVVL 232sets the lift amount of the intake valve 21 to be larger than the liftamount in the low-load range 415. The close timing of the intake valve21 is substantially the same as the close timing in the low-load range415. The open timing of the intake valve 21 is advanced from the opentiming in the low-load range 415. According to this open-and-close modeof the intake valve 21 and the exhaust valve 22, the amount of fresh airintroduced into the cylinder 11 increases and the introducing amount ofburnt gas decreases. The G/F of the mixture gas is 35:1, for example.

The injector 6 injects fuel into the cylinder 11 during the compressionstroke (see 717). The injector 6 may perform the injection all at onceas illustrated in FIG. 5 . Similarly to the retarded SI combustion,retarding the fuel injection is advantageous to reduce abnormalcombustion. Note that for example, when the engine 1 operates at a lowload in the first middle-load range 416, fuel may be injected duringeach of the intake stroke and the compression stroke.

The first spark plug 251 and the second spark plug 252 both ignite themixture gas near the compression TDC (see 718). The mixture gas startsthe flame propagation combustion near the compression TDC after theignition by the first spark plug 251 and the second spark plug 252. Theheat generated by the flame propagation combustion increases thetemperature inside the cylinder 11, and the flame propagation increasesthe pressure inside the cylinder 11. Accordingly, unburnt mixture gasself-ignites, for example, after the compression TDC, and starts the CIcombustion. After the start of the CI combustion, the flame propagationcombustion and the CI combustion progress in parallel. The waveform ofthe heat release rate may have two peaks as illustrated in FIG. 5 (see719).

Variations in the temperature inside the cylinder 11 before the start ofthe compression can be reduced by controlling the heat release amount inthe flame propagation combustion. The heat release amount in the flamepropagation combustion can be adjusted by the ECU 10 controlling theignition timing. Accordingly, the mixture gas self-ignites at a targettiming. In the SPCCI combustion, the ECU 10 controls the timing of thecompression ignition by controlling the ignition timing. Since the sparkignition controls the compression ignition in this combustion mode, thiscombustion mode may be referred to as the “SPark Controlled CompressionIgnition (SPCCI) combustion.”

5. MPCI Combustion

When the engine 1 operates in the second middle-load range 417, the ECU10 combusts the mixture gas inside the cylinder 11 by compressionignition. In more detail, the exhaust S-VT 241 sets the open and closetimings of the exhaust valve 22 to given timings. The exhaust VVL 242opens and closes the exhaust valve 22 twice. Internal EGR gas isintroduced into the cylinder 11. The intake CVVL 232 sets the liftamount of the intake valve 21 to be smaller than the lift amount in thefirst middle-load range 416. The close timing of the intake valve 21 issubstantially the same as the close timing in the first middle-loadrange 416. The open timing of the intake valve 21 is retarded from theopen timing in the first middle-load range 416 (see 720 and 724).According to this open-and-close mode of the intake valve 21 and theexhaust valve 22, the amount of fresh air introduced into the cylinder11 decreases and the introducing amount of burnt gas increases. The G/Fof the mixture gas is between 35:1 and 38:1, for example.

The injector 6 injects fuel into the cylinder 11 during each of theintake stroke and the compression stroke. The injector 6 performs splitinjection. In the second middle-load range 417, the ECU 10 changes theinjection mode between a squish injection and a trigger injection. Thesquish injection is a mode in which fuel is injected during the intakestroke and during the middle period of the compression stroke (see 721and 722) (e.g., a first injection mode). The trigger injection is a modein which fuel is injected during the intake stroke and during the endperiod of the compression stroke (see 725 and 726) (e.g., a secondinjection mode).

The squish injection slows down the CI combustion. As described above,fuel injected during the intake stroke is spread inside the cylinder 11by the strong intake flow and the homogeneous mixture gas is formedinside the cylinder 11. As illustrated in the lower part of FIG. 2 ,fuel injected in the middle period of the compression stroke reaches asquish area 171 outside of the cavity 31. The squish area 171 is low intemperature since it is located near a cylinder liner, and thetemperature further drops due to latent heat during vaporization of fuelspray. The temperature inside the cylinder 11 locally drops, and themixture gas becomes inhomogeneous inside the cylinder 11. As a result,for example, when the in-cylinder temperature is high, the mixture gasis compressed and ignited at a desired timing while reducing abnormalcombustion (see 723). The squish injection allows comparatively slow CIcombustion.

Each shaded rectangle in FIG. 5 indicates the injection period of theinjector 6, and the area of the rectangle corresponds to the injectionamount of fuel. In the squish injection, the injection amount of fuelduring the compression stroke is larger than the injection amount offuel during the intake stroke. Since the fuel is injected toward a largearea outside of the cavity 31, generation of smoke can be reduced evenwhen the amount of fuel is large. The temperature decreases as theamount of fuel increases. The injection amount of fuel during thecompression stroke may be set to an amount capable of achieving ademanded decrease in the temperature.

The trigger injection accelerates the CI combustion. As described above,the fuel injected during the intake stroke is spread inside the cylinder11 by the strong intake flow and the homogeneous mixture gas is formedinside the cylinder 11. As illustrated in FIG. 6 , the fuel injected inthe end period of the compression stroke is difficult to spread due tothe high pressure, and stays in an area inside the cavity 31. Note thatthe “area inside the cavity 31” means an area inward of an outerperipheral edge of the cavity 31 in the radial direction of the cylinder11. The internal part of the cavity 31 dented from the top surface ofthe piston 3 is also included in the area inside of the cavity 31. Themixture gas inside the cylinder 11 is inhomogeneous. Moreover, thetemperature at the central part of the cylinder 11 is high since it isfar from the cylinder liner. Since a lump of mixture gas at a high fuelconcentration is formed in the area at the high temperature, thecompression ignition of the mixture gas is accelerated. As a result, forexample, when the G/F of the mixture gas is high, the mixture gas ispromptly compressed and ignited after the fuel injection during thecompression stroke (see 727), and the CI combustion can be accelerated.The trigger injection enhances combustion stability.

In the trigger injection, the injection amount of fuel during thecompression stroke is smaller than the injection amount of fuel duringthe intake stroke. Since the fuel injection during the compressionstroke is performed in the end period of the compression stroke, theinjected fuel stays inside the cavity 31 and is difficult to spread.Reducing the fuel amount can suppress the generation of smoke. Theinjection amount of fuel during the compression stroke can be set to anamount capable of achieving both of the demanded acceleration of thecompression ignition and the reduction in the generation of smoke.

The squish injection and the trigger injection both make the mixture gasinside the cylinder 11 inhomogeneous. In this respect, it is differentfrom the HCCI combustion in which the homogeneous mixture gas is formed.Both of the squish injection and the trigger injection can control thetiming of the compression ignition by forming the inhomogeneous mixturegas.

Since the injector 6 injects fuel a plurality of times in thiscombustion mode, this mode may be referred to as the “Multiple Premixedfuel injection Compression Ignition (MPCI) combustion.”

Note that as illustrated in the second base map 402 in FIG. 4 , thehomogeneous SI combustion or the SPCCI combustion is performed in thethird range when the engine 1 is cold (the third range corresponds tothe range of the first base map 401 for the warm state, in which thecombustion modes are the HCCI, the MPCI, and the SPCCI). This is becausethe CI combustion becomes instable when the temperature of the engine 1is low. After the start-up of the engine 1, the ECU 10 changes the basemap from the second base map 402 for the cold state to the first basemap 401 for the warm state as the coolant temperature rises. When thebase map is changed, the ECU 10 may change the combustion mode, forexample, from the homogeneous SI combustion to the HCCI combustion evenwhen the speed and the load of the engine 1 do not change.

(Details of Engine Control According to Engine Load)

Here, in the timing charts of the combustion modes illustrated in FIG. 5, the combustion modes illustrated in the lower part of the figure arethe modes when the load of the engine 1 is lower, and the combustionmodes illustrated in the upper part of the figure are the modes when theengine load is higher. The G/F of the mixture gas is low when the engineload is high. On the other hand, the G/F of the mixture gas is high whenthe engine load is low. The amount of fresh air introduced into thecylinder 11 is small and the amount of burnt gas is large.

Next, the injection timings of fuel corresponding to the change in theengine load are compared. Here, an injection center of gravity relatedto the injection timing of fuel is defined. FIG. 7 is a viewillustrating the injection center of gravity. The horizontal axis inFIG. 7 indicates the crank angle, and the crank angle progresses fromleft to right in the figure. The injection center of gravity is thecenter of mass of fuel injected in one cycle with respect to the crankangle. The injection center of gravity is defined based on the injectiontiming and the injection amount of fuel in one cycle. Chart 71 in FIG. 7illustrates an injection timing soi_1 (start of injection) and aninjection period pw_1 in a case where the fuel is injected all at once(first injection). A left end of each rectangle in FIG. 7 indicates astart timing of the injection, a right end indicates an end timing ofthe injection, and a length between the left and right ends of therectangle indicates the injection period. The injection pressure of fuelis constant during one combustion cycle. Therefore, the injection amountis in proportion to the injection period. The injection amount may besubstituted by the injection period when the injection center of gravityis calculated.

An injection center of gravity ic_g when fuel is injected all at oncecoincides with a crank angle ic_1 at the middle of the one injectionperiod. The crank angle ic_1 (i.e., the injection center of gravityic_g) can be represented by the following Equation (1) based on theinjection start timing soi_1, the injection period pw_1, and a speed Neof the engine 1.ic_1=soi_1+(pw_1*Ne*360/60)/2=soi_1+3*pw_1*Ne  (1)Chart 72 in FIG. 7 illustrates a case where the start timing of theinjection is retarded from the case in chart 71. Since fuel is injectedall at once also in chart 72, the injection center of gravity can becalculated based on Equation (1). In the case where fuel is injected allat once, the injection center of gravity is retarded as the start timingof the injection is retarded.

Note that although illustration is omitted, the injection center ofgravity changes when the injection start timing is the same and theinjection period changes.

Chart 73 in FIG. 7 illustrates a case of the split injection. Theinjection timing and the injection period of the first injection inchart 73 are the same as the injection timing and the injection periodof the first injection in chart 71. A start timing of a second injectionis later than the start timing of the first injection.

When the injection includes two injections (first and secondinjections), since the injection center of gravity ic_g is the center ofmass of fuel injected in one cycle with respect to the crank angle, theinjection center of gravity ic_g is defined on the basis of thefollowing Equation (2).ic_g=(pw_1*ic_1+pw_2*ic_2)/(pw_1+pw_2)  (2)The “ic_1” can be calculated based on Equation (1). Similarly, “ic_2”can be calculated based on the following Equation (3).ic_2=soi_2+(pw_2*Ne*360/60)/2=soi_2+3*pw_2*Ne  (3)On the basis of Equations (1), (2), and (3), the injection center ofgravity ic_g can be calculated based on the following Equation (4).ic_g=(pw_1*(soi_1+3*pw_1*Ne)+pw_2*(soi_2+3*pw_2*Ne))/(pw_1+pw_2)  (4)Since the second injection is added to the first injection in chart 73in FIG. 7 , the injection center of gravity ic_g in chart 73 is retardedfrom the injection center of gravity ic_g in chart 71.

Note that when Equation (4) is generalized and the injector 6 injectsfuel “n” times in one cycle, the injection center of gravity ic_g can becalculated based on the following Equation (5).ic_g=(pw_1*(soi_1+3*pw_1*Ne)+ . . . +pw_n*(soi_n+3*pw_n*Ne))/(pw_1+ . .. +pw_n)  (5)As illustrated in FIG. 5 , the G/F of the mixture gas is high (e.g.,G/F=40:1) when the load of the engine 1 is low. The injector 6 injectsfuel during the intake stroke. The injection center of gravity is on theadvanced side. When the load of the engine 1 is higher, the G/F of themixture gas is lower (e.g., G/F=35:1 or 38:1). The injector 6 injectsfuel during the intake stroke and during the compression stroke (see721, 722, 725, and 726). The injection center of gravity is relativelyretarded.

When the load of the engine 1 is further higher, the G/F of the mixturegas is further lower (e.g., G/F=35:1). The injector 6 injects fuelduring the compression stroke (see 717). The injection center of gravityis further retarded relatively.

When the load of the engine 1 is further higher, the G/F of the mixturegas is further lower (e.g., G/F=20:1 or 25:1). The injector 6 injectsfuel during the intake stroke (see 702), or during the compressionstroke (see 706 and 710). The injection center of gravity is relativelyadvanced, or relatively retarded.

When comparing the HCCI combustion with the homogeneous SI combustionand the retarded SI combustion, the G/F of the mixture gas is higher inthe HCCI combustion, and the G/F of the mixture gas is lower in thehomogeneous SI combustion and the retarded SI combustion. Suppose thatthe engine 1 is an engine which switches only between the HCCIcombustion, and the homogeneous SI combustion or the retarded SIcombustion. In this case, when the combustion mode is switchedcorresponding to the change in the load of the engine 1, the G/F of themixture gas needs to be changed largely. However, the responsivity ofthe variable valve operating device including the intake S-VT 231, theintake CVVL 232, the exhaust S-VT 241, and the exhaust VVL 242 is not sohigh. Therefore, it is difficult to instantly change the G/F of themixture gas.

In the MPCI combustion and the SPCCI combustion, the G/F of the mixturegas is between the G/F for the HCCI combustion and the G/F for the SIcombustion (i.e., at the middle G/F). The G/F can be changed promptlybetween the HCCI combustion, and the MPCI combustion or the SPCCIcombustion, and between the SI combustion, and the MPCI combustion orthe SPCCI combustion.

As will be described later in detail, in the MPCI combustion and theSPCCI combustion, the injection center of gravity is retarded from theinjection center of gravity in the HCCI combustion. Therefore, the MPCIcombustion and the SPCCI combustion are the modes capable of securingcombustion stability and reducing abnormal combustion when the mixturegas is at the middle G/F. This engine 1 can seamlessly switch thecombustion mode between the SI combustion, the HCCI combustion, the MPCIcombustion, and the SPCCI combustion by promptly changing the G/F of themixture gas corresponding to the change in the engine load. As a result,securing combustion stability and reducing abnormal combustion can beachieved over the entire load range of the engine 1.

Note that in the MPCI combustion, the injector 6 injects fuel during theintake stroke and during the compression stroke. When the G/F of themixture gas is between the G/F for the HCCI combustion and the G/F forthe SI combustion, the injector 6 may inject fuel all at once such thatthe injection center of gravity is retarded from the injection center ofgravity in the HCCI combustion, instead of the split injection. When theinjection center of gravity is retarded, a period of time from the fuelinjection to the ignition becomes shorter, and thus, the mixture gasinside the cylinder 11 does not become homogeneous. Such inhomogeneousmixture gas enables the securing of combustion stability and thereduction in abnormal combustion at the middle G/F.

(Modifications of Open-and-close Mode of Intake Valve and Exhaust Valve)

Although FIG. 5 illustrates the configuration in which the exhaust VVL242 opens the exhaust valve 22 during each of the exhaust stroke and theintake stroke, the configuration of the variable valve operating deviceis not limited to the configuration. Next, modifications of the variablevalve operating device are described with reference to FIG. 8 .

In FIG. 8 , “81” illustrates lift curves of the exhaust valve 22, whichare different from the lift curves illustrated in FIG. 5 . A lift curve811 in the homogeneous SI combustion, a lift curve 812 in the secondretarded SI combustion, a lift curve 813 in the first retarded SIcombustion are the same as the lift curves 701, 705, and 709 in FIG. 5 ,respectively. A lift curve 814 in the SPCCI combustion, a lift curve 815in the MPCI combustion, and a lift curve 816 in the HCCI combustion aredifferent from the lift curves 716, 720, 724, and 713 in FIG. 5 . Asindicated by 814, 815, and 816 in FIG. 8 , after the exhaust valve 22 isopened during the exhaust stroke and the lift amount gradually decreasesfrom the maximum lift, the exhaust valve 22 is not closed and maintainsa given lift amount. The exhaust valve 22 is not closed until a giventiming after the intake TDC during the intake stroke. Maintaining theopen state of the exhaust valve 22 without closing is advantageous forloss reduction of the engine 1. Note that lift curves of the intakevalve 21 in the lift curve 814 in the SPCCI combustion, the lift curve815 in the MPCI combustion, and the lift curve 816 in the HCCIcombustion are the same as the lift curves 716, 720, 724, and 713 inFIG. 5 , respectively.

In FIG. 8 , “82” illustrates still other lift curves of the exhaustvalve 22. In this modification, the variable valve operating device isnot provided with the intake CVVL 232 and the exhaust VVL 242. Thevariable valve operating device is provided with the intake S-VT 231 andthe exhaust S-VT 241, and changes the open and close timings of theintake valve 21 and the exhaust valve 22.

As indicated by 823, 824, 825, and 826, a negative overlap period duringwhich both of the intake valve 21 and the exhaust valve 22 are closedhaving the intake TDC therebetween, is provided so that internal EGR gasremains inside the cylinder 11. That is, the exhaust valve 22 is closedbefore the intake TDC.

When the load of the engine 1 decreases and the amount of burnt gasintroduced into the cylinder 11 is to be increased, the close timing ofthe exhaust valve 22 advances. Moreover, when the amount of fresh airintroduced into the cylinder 11 is to be reduced, the close timing ofthe intake valve 21 retards after an intake bottom dead center (BDC) tobe separated therefrom. The negative overlap period is increased as theload of the engine 1 is lower.

Note that the variable valve operating device may provide a positiveoverlap period, such as at 821 and 822, during which both of the intakevalve 21 and the exhaust valve 22 are opened having the intake TDCtherebetween so that internal EGR gas is reintroduced into the cylinder11.

(Determination of Combustion Mode)

The ECU 10 determines the operating state of the engine 1 based on themeasurement signals of the sensors SW1 to SW10 described above. The ECU10 controls the intake S-VT 231, the intake CVVL 232, the exhaust S-VT241, and the exhaust VVL 242 according to the determined operatingstate. The intake S-VT 231, the intake CVVL 232, the exhaust S-VT 241,and the exhaust VVL 242 control the opening and closing of the intakevalve 21 and the exhaust valve 22 based on the control signals receivedfrom the ECU 10. Accordingly, the filling amount of intake air insidethe cylinder 11 is adjusted. In more detail, the amount of fresh air andburnt gas introduced into the cylinder 11 is adjusted.

The ECU 10 also adjusts the injection amount and timing of fuelaccording to the operating state of the engine 1. The injector 6 injectsfuel into the cylinder 11 in a specified amount at a specified timingbased on the control signal received from the ECU 10.

The ECU 10 also controls the first spark plug 251 and the second sparkplug 252 according to the operating state of the engine 1. The firstspark plug 251 and the second spark plug 252 ignite the mixture gas at aspecified timing based on the control signal received from the ECU 10.The ECU 10 may not output the control signal to the first spark plug 251and the second spark plug 252. In this case, the first spark plug 251and the second spark plug 252 are inhibited from igniting the mixturegas.

As described above, the engine 1 operates while switching the combustionmode between the plurality of types of combustion modes according to theoperating state of the engine 1. Therefore, securing combustionstability and reducing abnormal combustion can be achieved over theentire wide operation range.

FIG. 9 illustrates a relationship between the G/F of the mixture gas andan in-cylinder temperature T_(IVC) in each combustion mode, at which thesecuring of combustion stability and the reduction in abnormalcombustion are achieved. To be accurate, the in-cylinder temperatureT_(IVC) is an in-cylinder temperature when the intake valve 21 isclosed. Moreover, FIG. 9 illustrates an example when the speed of theengine 1 is 2,000 rpm, and an IMEP (Indicated Mean Effective Pressure)is about 400 kPa.

1. Homogeneous SI Combustion

The homogeneous SI combustion can secure combustion stability and reduceabnormal combustion when the G/F is relatively low. As the G/F increases(i.e., as the G/F becomes leaner), the combustion period of the mixturegas becomes longer. Even if the ignition timing is advanced to shortenthe combustion period, combustion stability cannot be secured when theG/F is too high. That is, the maximum G/F at which the homogeneous SIcombustion is possible exists (see a solid line in FIG. 9 ).

Moreover, when the T_(IVC) becomes high due to the increase in theinternal EGR gas, the combustion period becomes longer as a result ofthe deceleration in the combustion. The combustion period can beshortened by advancing the ignition timing until the T_(IVC) reaches acertain temperature. When the T_(IVC) is further increased, abnormalcombustion is likely to be caused. Even if the ignition timing isretarded to reduce abnormal combustion, the ignition timing becomes toolate when the T_(IVC) becomes too high, and thus, combustion stabilitycannot be secured. That is, the maximum in-cylinder temperature T_(IVC)at which the homogeneous SI combustion is possible exists.

2. HCCI Combustion

The HCCI combustion can secure combustion stability and reduce abnormalcombustion when the G/F is relatively high and the in-cylindertemperature T_(IVC) is relatively high. As the G/F decreases (i.e., asthe G/F becomes richer), the CI combustion becomes too intense, whichleads to abnormal combustion. Even if the T_(IVC) is lowered to retardthe ignition timing and decelerate the combustion, combustion stabilitydegrades when the T_(IVC) becomes too low. That is, the minimum G/F andthe minimum in-cylinder temperature T_(IVC) at which the HCCI combustionis possible exist (see a thicker solid line in FIG. 9 ).

As is apparent from FIG. 9 , the “G/F-T_(IVC) range” where thehomogeneous SI combustion is possible, and the “G/F-T_(IVC) range” wherethe HCCI combustion is possible are separated from each other. Asdescribed above, suppose that the engine 1 switches only between thehomogeneous SI combustion and the HCCI combustion corresponding to thechange in the load of the engine 1, the G/F of the mixture gas and thein-cylinder temperature T_(IVC) need to be changed largely correspondingto the switching of the combustion mode. The G/F of the mixture gas andthe in-cylinder temperature T_(IVC) are adjusted mainly by theadjustment of the filling amount of intake air. However, it is difficultto instantly change the G/F of the mixture gas and the in-cylindertemperature T_(IVC) corresponding to the switching of the combustionmode, because of a response delay of the intake S-VT 231, the intakeCVVL 232, the exhaust S-VT 241, and the exhaust VVL 242.

3. Retarded SI Combustion

As described above, when the G/F of the mixture gas is made leaner, orthe in-cylinder temperature T_(IVC) is made higher than the operablerange of the homogeneous SI combustion, combustion stability cannot besecured. In the retarded SI combustion, as described above, the injector6 injects fuel into the cylinder 11 near the compression TDC, that is,before the ignition by the first spark plug 251 and the second sparkplug 252. Since the fuel is not injected into the cylinder 11 untilimmediately before the ignition, preignition can be avoided.

The injection of fuel near the compression TDC causes the flow insidethe cylinder 11, and after the ignition by the first spark plug 251 andthe second spark plug 252, the flame is promptly propagated by the flow.Accordingly, the rapid combustion is achieved, and combustion stabilitycan be secured while reducing knocking. In the “G/F-T_(IVC) range” wherethe retarded SI combustion is possible, the G/F of the mixture gas ishigher than that in the “G/F-T_(IVC) range” where the homogeneous SIcombustion is possible (see a broken line in FIG. 9 ). The retarded SIcombustion extends its operable range in the leaner-G/F side compared tothe homogeneous SI combustion.

4. SPCCI Combustion

When the G/F of the mixture gas is made further leaner, or thein-cylinder temperature T_(IVC) is made further higher than the operablerange of the retarded SI combustion, gentle CI combustion (differentfrom knocking) starts after the flame propagation combustion started bythe ignition of the first spark plug 251 and the second spark plug 252.In the SPCCI combustion including the controlled CI combustion, the G/Fis higher than the “G/F-T_(IVC) range” where the retarded SI combustionis possible (see a one-dot chain line in FIG. 9 ). The SPCCI combustionextends its operable range in the leaner-G/F side compared to thehomogeneous SI combustion and the retarded SI combustion. However, alarge gap still exists between the “G/F-T_(IVC) range” of the SPCCIcombustion and the “G/F-T_(IVC) range” of the HCCI combustion.

5. MPCI Combustion

The MPCI combustion extends its operable range in the richer-G/F sideand the lower-T_(IVC) side, compared to the operable range of the HCCIcombustion.

First, when the G/F of the mixture gas is made richer than the operablerange of the HCCI combustion, the CI combustion becomes intense, whichleads to abnormal combustion. In order to decelerate the CI combustion,fuel is injected into the cylinder 11 in the middle period of thecompression stroke in the squish injection of the MPCI combustion. Asdescribed above, the injected fuel reaches the squish area 171 outsideof the cavity 31, and locally increases the fuel concentration at thesquish area 171 and decreases the temperature. As a result, the timingof the compression ignition is retarded, and the combustion is sloweddown. The squish injection extends its operable range mainly in thericher-G/F side compared to the operable range of the HCCI combustion.

Next, when the T_(IVC) is made lower than the operable range of the HCCIcombustion, the compression ignition timing retards and the combustionbecomes too slow, which lowers combustion stability. In order to advancethe compression ignition timing, fuel is injected into the cylinder 11in the end period of the compression stroke in the trigger injection ofthe MPCI combustion. As described above, the injected fuel does notspread and forms the lump of mixture gas at a high fuel concentrationinside the cavity 31. As a result, the compression ignition startspromptly after the fuel injection, and the surrounding homogeneousmixture gas also promptly combusts by self-ignition. The triggerinjection extends its operable range mainly in the lower-T_(IVC) sidecompared to the operable range of the HCCI combustion.

Part of the “G/F-T_(IVC) range” of the MPCI combustion overlaps with the“G/F-T_(IVC) range” of the SPCCI combustion. The gap between the“G/F-T_(IVC) ranges” of the homogeneous SI combustion and the retardedSI combustion, and the “G/F-T_(IVC) range” of the HCCI combustion isfilled.

Here, the “G/F-T_(IVC) range” of the MPCI combustion is divided into theranges where the squish injection is performed and where the triggerinjection is performed (see a broken dividing line in FIG. 9 ). In therange where the squish injection is performed in the “G/F-T_(IVC) range”of the MPCI combustion, the G/F is relatively low and the T_(IVC) isrelatively high. On the other hand, in the range where the triggerinjection is performed in the “G/F-T_(IVC) range” of the MPCIcombustion, the G/F is relatively high and the T_(IVC) is relativelylow.

(Operation Control of Engine)

The ECU 10 adjusts the G/F of the mixture gas and the in-cylindertemperature T_(IVC) based on the base map illustrated in FIG. 4 suchthat the combustion mode corresponding to the demanded load and speed ofthe engine 1 is achieved.

However, the G/F of the mixture gas and/or the in-cylinder temperatureT_(IVC) may not correspond to the operating state of the engine 1, andmay be deviated from the target G/F and/or the target in-cylindertemperature T_(IVC) due to, for example, the response delay of thevariable valve operating device. When the G/F of the mixture gas and/orthe in-cylinder temperature T_(IVC) are deviated from the target G/Fand/or the target in-cylinder temperature T_(IVC), the mixture gascannot be combusted in the intended combustion mode, which may lowercombustion stability and/or cause abnormal combustion. In this respect,the ECU 10 temporarily sets the combustion mode according to theoperation state of the engine 1, determines the target G/F and/or thetarget in-cylinder temperature T_(IVC), and controls the variable valveoperating device. Moreover, the ECU 10 switches the combustion modeaccording to an actual G/F and/or an actual in-cylinder temperatureT_(IVC) (accurately, an estimated G/F and/or an estimated in-cylindertemperature T_(IVC)), and adjusts the injection timing of fuel andwhether or not to perform the ignition.

FIG. 10 illustrates a selection map related to the operation control ofthe engine 1. FIG. 10 is an enlarged view of the third range in thefirst base map 401 of FIG. 4 , where the HCCI combustion is performed(i.e., the low-load range 415). The low-load range 415 is defined basedon the speed and the load of the engine 1. As illustrated in FIG. 10 ,the low-load range 415 is further subdivided based on the load and speedof the engine 1. Although in the selection map of FIG. 10 the low-loadrange 415 is subdivided into nine ranges as one example, the number ofsubdivided ranges is not limited in particular. Note that althoughillustration is omitted, such a selection map is set for each range inthe base map of FIG. 4 .

The “G/F-T_(IVC) range” corresponding to FIG. 9 is set for eachsubdivided range in the low-load range 415. As described above, the“G/F-T_(IVC) range” defines the combustion mode based on the G/F of themixture gas and the in-cylinder temperature T_(IVC). The ECU 10 sets(temporarily sets) the combustion mode based on the base map of FIG. 4according to the demanded load and speed of the engine 1, and adjuststhe filling amount of intake air. Furthermore, the ECU 10 conclusivelydetermines the combustion mode based on the selection map of FIG. 10according to the demanded load and speed, and the estimated G/F andin-cylinder temperature T_(IVC).

Here, as illustrated in FIG. 10 , the “G/F-T_(IVC) range” variesaccording to the load and speed of the engine 1. When the speed is high,the HCCI combustion, the MPCI combustion, and the SPCCI combustion arepossible even when the in-cylinder temperature is high. On the otherhand, when the speed is low, the HCCI combustion and the MPCI combustionare possible only when the in-cylinder temperature is low.

Moreover, when comparing the ranges at the same load, the “G/F-T_(IVC)range” of the SPCCI combustion increases and the “G/F-T_(IVC) range” ofthe retarded SI combustion decreases, as the speed increases. On thecontrary, the “G/F-T_(IVC) range” of the SPCCI combustion decreases andthe “G/F-T_(IVC) range” of the retarded SI combustion increases as thespeed decreases.

Moreover, when comparing the ranges at the same speed, in both of the“G/F-T_(IVC) ranges” of the HCCI combustion and the MPCI combustion, theminimum in-cylinder temperature T_(IVC) moves to the higher-temperatureside as the load decreases.

Next, process of operation control of the engine 1, executed by the ECU10 is described with reference to FIG. 11 . First, at step S1, the ECU10 acquires the measurement signals of the various sensors, and next atstep S2, the ECU 10 calculates a target torque Tq (or a target load)based on the engine speed Ne and an accelerator opening APO.

At step S3, the ECU 10 selects the first base map 401 or the second basemap 402 illustrated in FIG. 4 based on the temperature of the coolant ofthe engine 1, and temporarily determines the combustion mode based onthe calculated target torque Tq and the engine speed Ne, and theselected base map.

At step S4, the ECU 10 calculates, based on the operating state of theengine 1, a target valve timing VT and a target valve lift VL for eachof the intake valve 21 and the exhaust valve 22. The target valve liftVL includes the valve lift of the intake valve 21 which is continuouslychanged by the intake CVVL 232, and the cam of the exhaust valve 22switched by the exhaust VVL 242. Moreover, at step S4, the ECU 10calculates a target amount of fuel injection Qf.

At step S5, the ECU 10 outputs the control signals to the intake S-VT231, the intake CVVL 232, the exhaust S-VT 241, and the exhaust VVL 242to achieve the target valve timing VT and the target valve lift VL.

At step S6, the ECU 10 detects an actual valve timing VT and an actualvalve lift VL of the intake valve 21, and an actual valve timing VT andan actual valve lift VL of the exhaust valve 22, based on themeasurement signals of the intake cam-angle sensor SW8, the exhaustcam-angle sensor SW9, and the intake cam-lift sensor SW10.

At step S7, the ECU 10 estimates the amount of burnt gas (EGR amount)and fresh air introduced into the cylinder 11 based on the actual valvetiming VT and valve lift VL, an air temperature Tair, and a coolanttemperature Thw of the engine 1.

Then, at step S8, the ECU 10 estimates the G/F of the mixture gas andthe in-cylinder temperature T_(IVC) based on the fuel injection amountQf, and the amount of burnt gas and fresh air estimated at step S7.

At step 9, the ECU 10 determines the combustion mode in accordance withthe selection map illustrated in FIG. 10 as one example, based on theG/F and the in-cylinder temperature T_(IVC) estimated at step S8. Then,at step S10, the ECU 10 determines an ignition timing IGT and theinjection pattern (i.e., the injection timing) corresponding to thedetermined combustion mode.

At step S11, the ECU 10 outputs the control signal to the injector 6.The injector 6 injects fuel based on the determined injection pattern.Moreover, when the ignition is to be performed, the ECU 10 also outputsthe control signal to the first spark plug 251 and the second spark plug252. The first spark plug 251 and the second spark plug 252 ignite themixture gas.

According to the flowchart in FIG. 11 , when the ECU 10 changes the G/Fof the mixture gas according to the demanded engine torque, the ECU 10can set the timing of fuel injection by the injector 6 in considerationof the response delay of the variable valve operating device. Since themixture gas combusts in the mode suitable for the state inside thecylinder 11, the engine 1 can meet the standard of combustion stabilityand reduce abnormal combustion.

Note that the present disclosure is applicable not only to the enginewith the configuration described above, but to engines with variousconfigurations.

It should be understood that the embodiments herein are illustrative andnot restrictive, since the scope of the invention is defined by theappended claims rather than by the description preceding them, and allchanges that fall within metes and bounds of the claims, or equivalenceof such metes and bounds thereof, are therefore intended to be embracedby the claims.

DESCRIPTION OF REFERENCE CHARACTERS

-   -   1 Engine    -   10 ECU (Controller)    -   11 Cylinder    -   21 Intake Valve    -   22 Exhaust Valve    -   231 Intake S-VT (Variable Valve Operating Device)    -   232 Intake CVVL (Variable Valve Operating Device)    -   241 Exhaust S-VT (Variable Valve Operating Device)    -   242 Exhaust VVL (Variable Valve Operating Device)    -   251 First Spark Plug    -   252 Second Spark Plug    -   3 Piston    -   31 Cavity    -   6 Injector

What is claimed is:
 1. An engine system, comprising: an engine having acylinder and a piston reciprocatably accommodated in the cylinder; aninjector attached to the engine and configured to inject fuel into thecylinder; a spark plug attached to the engine and configured to ignite amixture gas of fuel and intake air, the intake air containing fresh airand burnt gas; a variable valve operating device connected to an intakevalve and an exhaust valve, and configured to control opening andclosing of the intake valve and the exhaust valve to adjust a fillingamount of the intake air; and a controller electrically connected to theinjector, the spark plug, and the variable valve operating device, andconfigured to control the injector, the spark plug, and the variablevalve operating device according to a demanded load of the engine,wherein when the engine operates at a given speed and the demandedengine load is a first load, the controller controls the injector andthe variable valve operating device to make a mass ratio (G/F) of theintake air inside the cylinder to fuel be at a first G/F, and controlsthe spark plug so that the mixture gas inside the cylinder combusts byflame propagation, wherein when the engine operates at the given speedand the demanded engine load is a second load lower than the first load,the controller controls the injector and the variable valve operatingdevice to make the mass ratio be at a second G/F higher than the firstG/F, and controls the injector to make an injection center of gravity beat a first timing so that the entire mixture gas inside the cylindercombusts by compression ignition, the injection center of gravity beingdefined based on an injection timing and an injection amount of fuelduring one cycle, wherein when the engine operates at the given speedand the demanded engine load is lower than the first load and higherthan the second load, the controller controls the injector and thevariable valve operating device to make the mass ratio be at a third G/Fhigher than the first G/F and lower than the second G/F, and controlsthe injector to make the injection center of gravity be at a secondtiming later than the first timing so that at least part of the mixturegas inside the cylinder combusts by compression ignition, and whereinwhen the engine operates at the given speed and the demanded engine loadis lower than the first load and higher than the second load, thecontroller switches between a first injection mode and a secondinjection mode based on an estimated G/F, and an estimated temperatureinside the cylinder at a close timing of the intake valve, the firstinjection mode being a mode in which the controller controls theinjector to inject fuel during each of an intake stroke and a middleperiod of a compression stroke, and the second injection mode being amode in which the controller controls the injector to inject fuel duringeach of the intake stroke and an end period of the compression stroke.2. The engine system of claim 1, wherein when the engine operates at thegiven speed and the demanded engine load is lower than the first loadand higher than the second load, the controller inhibits the operationof the spark plug so that the entire mixture gas inside the cylindercombusts by compression ignition.
 3. The engine system of claim 1,wherein when the engine operates at the given speed and the demandedengine load is lower than the first load and higher than the secondload, the controller actuates the spark plug so that at least part ofthe mixture gas inside the cylinder combusts by flame propagation, andthe remaining mixture gas combusts by compression ignition.
 4. Theengine system of claim 1, wherein when the engine operates at the givenspeed and the demanded engine load is the second load, the controllercontrols the injector to inject fuel during the intake stroke, andwherein, when the engine operates at the given speed and the demandedengine load is lower than the first load and higher than the secondload, the controller controls the injector to inject fuel during each ofthe intake stroke and the compression stroke.
 5. The engine system ofclaim 1, wherein when the engine operates at the given speed and thedemanded engine load is the first load, the controller controls theinjector to inject fuel during the intake stroke.
 6. The engine systemof claim 1, wherein when the engine operates at the given speed and thedemanded engine load is the first load, the controller controls theinjector to inject fuel in a latter half of the compression stroke. 7.The engine system of claim 1, wherein the variable valve operatingdevice controls the opening and closing of the intake valve and theexhaust valve so that the burnt gas remains inside the cylinder, or theburnt gas is introduced into the cylinder through the intake valve orthe exhaust valve.
 8. The engine system of claim 1, wherein a geometriccompression ratio of the engine is 15:1 or above and 30:1 or below.
 9. Aengine system, comprising: an engine having a cylinder and a pistonreciprocatably accommodated in the cylinder; an injector attached to theengine and configured to inject fuel into the cylinder; a spark plugattached to the engine and configured to ignite a mixture gas of fueland intake air, the intake air containing fresh air and burnt gas; avariable valve operating device connected to an intake valve and anexhaust valve, and configured to control opening and closing of theintake valve and the exhaust valve to adjust a filling amount of theintake air; and a controller electrically connected to the injector, thespark plug, and the variable valve operating device, and configured tocontrol the injector, the spark plug, and the variable valve operatingdevice according to a demanded load of the engine, wherein when theengine operates at a given speed and the demanded engine load is a firstload, the controller controls the injector and the variable valveoperating device to make a mass ratio (G/F) of the intake air inside thecylinder to fuel be at a first G/F, and controls the spark plug so thatthe mixture gas inside the cylinder combusts by flame propagation,wherein when the engine operates at the given speed and the demandedengine load is a second load lower than the first load, the controllercontrols the injector and the variable valve operating device to makethe mass ratio be at a second G/F higher than the first G/F, andcontrols the injector to make an injection center of gravity be at afirst timing so that the entire mixture gas inside the cylinder combustsby compression ignition, the injection center of gravity being definedbased on an injection timing and an injection amount of fuel during onecycle, wherein when the engine operates at the given speed and thedemanded engine load is lower than the first load and higher than thesecond load, the controller controls the injector and the variable valveoperating device to make the mass ratio be at a third G/F higher thanthe first G/F and lower than the second G/F, and controls the injectorto make the injection center of gravity be at a second timing later thanthe first timing so that at least part of the mixture gas inside thecylinder combusts by compression ignition, wherein when the engineoperates at the given speed and the demanded engine load is lower thanthe first load and higher than the second load, the controller inhibitsthe operation of the spark plug so that the entire mixture gas insidethe cylinder combusts by compression ignition, wherein when the engineoperates at the given speed and the demanded engine load is the secondload, the controller controls the injector to inject fuel during anintake stroke, wherein when the engine operates at the given speed andthe demanded engine load is lower than the first load and higher thanthe second load, the controller controls the injector to inject fuelduring each of the intake stroke and a compression stroke, and whereinwhen the engine operates at the given speed and the demanded engine loadis lower than the first load and higher than the second load, thecontroller switches between a first injection mode and a secondinjection mode based on an estimated G/F, and an estimated temperatureinside the cylinder at a close timing of the intake valve, the firstinjection mode being a mode in which the controller controls theinjector to inject fuel during each of the intake stroke and a middleperiod of the compression stroke, and the second injection mode being amode in which the controller controls the injector to inject fuel duringeach of the intake stroke and an end period of the compression stroke.10. The engine system of claim 9, wherein a cavity is formed in a topsurface of the piston, wherein in the first injection mode, thecontroller controls the injector to inject fuel to outside of the cavityin the middle period of the compression stroke such that an amount ofinjection during the compression stroke is larger than an amount ofinjection during the intake stroke, and wherein in the second injectionmode, the controller controls the injector to inject fuel to inside ofthe cavity in the end period of the compression stroke such that anamount of injection during the compression stroke is smaller than anamount of injection during the intake stroke.
 11. The engine system ofclaim 10, wherein when the engine operates at the given speed and thedemanded engine load is the first load, the controller controls theinjector to inject fuel during the intake stroke.
 12. The engine systemof claim 11, wherein when the engine operates at the given speed and thedemanded engine load is the first load, the controller controls theinjector to inject fuel in a latter half of the compression stroke. 13.The engine system of claim 12, wherein the variable valve operatingdevice controls the opening and closing of the intake valve and theexhaust valve so that the burnt gas remains inside the cylinder, or theburnt gas is introduced into the cylinder through the intake valve orthe exhaust valve.
 14. The engine system of claim 13, wherein ageometric compression ratio of the engine is 15:1 or above and 30:1 orbelow.